In a large mixing bowl, combine the ground turkey, breadcrumbs, beaten egg, minced garlic, Parmesan, Greek yogurt, parsley, onion powder, salt, and black pepper. Mix with your hands or a fork just until everything is evenly combined. Do not overmix or the meatballs will turn dense. The mixture will be soft and slightly sticky, which is normal for turkey.

Scoop and roll the mixture into meatballs about 1 1/2 inches across. You should get roughly 20 to 24 meatballs. Place them on a rimmed baking sheet as you go. Wetting your hands lightly between rolls keeps the meat from sticking to your palms.

Pour about half of the marinara sauce into the bottom of the slow cooker and spread it in an even layer. Arrange the raw meatballs on top of the sauce in a single layer, or as close to one as your cooker allows. Pour the remaining marinara over and around the meatballs. Sprinkle the red pepper flakes over the top.

Place the lid on and cook on LOW for 6 to 7 hours. The meatballs will cook through completely and the sauce will reduce and concentrate around them. At the 6-hour mark, gently press one meatball with a spoon. It should feel firm all the way through with no soft, raw-looking center.

Turn off the heat and scatter the torn fresh basil over the top. Let the meatballs rest in the sauce for 5 minutes before serving. Spoon meatballs and sauce over cooked spaghetti or zucchini noodles and finish each bowl with extra Parmesan.